The present invention relates to an electrical fuse and more particularly, to a thermal fuse for use in various electrical and electronic equipment so as to prevent the equipment from overheating.
The thermal or temperature fuse is an electrical safety device including a fusible material that melts by heat so as to interrupt functioning of a circuit when electric current passing therethrough becomes excessively large, and has been widely employed in various electrical and electronic equipment and appliances for protecting them against possible damage due to overheating.
Conventionally, as shown in FIG. 1(a), the known thermal fuse includes, for example, a cylindrical metallic casing 10 open at its one end 10a, a normally solid temperature-sensing fusable pellet 11 filled to a predetermined thickness, in the other end 10b of the casing 10 so as to be melted at a predetermined temperature for collapsing an electrically non-conductive bush member or end plug 14, for example, of ceramic material through which a lead wire or conductor 12 provided with a contact portion 12a at its forward end axially extends and which is filled into the open end 10a of the case 10 for closing said end 10a by application of sealing resin 13 thereonto a slidable contact member 15 having a configuration as shown in FIG. 1(b) formed by providing a plurality of notches n around a peripheral edge of a circular disc (not shown) to leave the corresponding number of resilient projections or blades 15a extending outwardly from a main portion 15b of the contact member 15 to be directed in one direction as shown and slidably disposed within the casing 10 between the fusible pellet 11 and bush member 14, with the blades 15a thereof contacting under pressure the inner peripheral surface of the casing 10 by the resiliency of the blades 15a for electrical conduction therebetween, a first compression spring 16a held under compression between the slidable contact member 15 and bush member 14, and a second compression spring 16b having spring force sufficiently larger than that of the first spring 16a and also held under compression between the contact member 15 and fusible pellet 11 through metallic discs 17a and 17b respectively.
In the conventional arrangement as described above, when the electrical appliance equipped with said thermal fuse is normally working without overheating, the slidable contact member 15 is caused to contact under pressure the contact portion 12a of the lead wire 12 by the urging force of the second compression spring 16b for electrical conduction of the lead wire 12 with another lead wire 18 extending outwardly from the other end or bottom 10b of the casing 10, through the metallic casing 10, contact member 15 and contact portion 12a. Meanwhile, when the electrical appliance is overheated due to some trouble, the heat generated thereby melts the fusible pellet 11 to collapse for releasing the second compression spring 16b from its compressed state as shown in FIG. 1(c), and thus, the contact member 15 is shifted to the side of the second compression spring 16b by the spring force of the first compression spring 16a to disconnect the contact member 15 from the contact portion 12a for cutting off the electrical conduction between the lead wires 12 and 18.
Although the prior art thermal fuse as described in the foregoing is capable of increasing the accuracy for rated temperature thereof with favorable characteristics in variation with time since the melting temperature of the fusible pellet 11 is extremely stable, friction caused by the sliding movement of the contact member 15 within the casing 10 tends to be increased especially when the thermal fuse is formed into a small size, and for achieving positive cutting off of electrical conduction between the lead wires 12 and 18 through smooth sliding movement of the first compression spring 16a upon melting of the fusible pellet 11, higher standards are required for the configuration, dimensions, materials, etc. of the slidable contact member 15, thus giving rise to difficulties in the manufacture of the slidable contact member or incorporation thereof into the metallic casing 10 or in the reduction of the diameter of the heat fuse.
Another disadvantage inherent in the known thermal fuse of the above described type is such that, since the lead wire 12 is fixed to the bush member 14 only by the sealing resin 13, if the bonding between the lead wire 12 and sealing resin 13 is imperfect, the lead wire 12 moves in the axial direction of the bush member 14 after the melting of the fusible pellet 11 to cause the contact portion 12a to again contact the slidable contact member 15 for undesirable electrical conduction between the lead wires 12 and 18.